Process for the preparation of potassium oxalate from potassium formate



Patented Mar. 3, 1936 UETED STATES PATENT OFFICE PROCESS FOR'THE PREPARATION OF PO- TASSIUM OXALATE FROM POTASSIUM FORMATE No Drawing. Application June 20, 1933, Serial No. 676,639V

17 Claims.

As is known, the oxalates of the alkali metals can be obtained by heating the corresponding alkali formates, a number of proposals having already been'made in connection with the preparation of sodium oxalate with a view to obtaining technically serviceable products. Thus, for example, it has been proposed to effect the heating of the formates in the presence of alkali hydroxides. The attempt to transfer to, or make useof the knowledge gained in these methods of producing sodium oxalate in, the preparation of potassium oxalate, however, unexpectedly resulted in complete failure, in that it was not possible to obtain yields greater than at most 75%.

Asa result of extended experiments it has now been found that these failures hitherto attending the preparation of potassium oxalate are attributable to the ignorance or'non-observance of the important part played on the one hand by the partial pressure of the hydrogen and,'on the other hand, by the simultaneous presence of alkali hydroxide and the regulation of the quantities thereof. 0

It has been found that a relationship exists between the reaction temperature-and the partial pressure of the hydrogen of such a kind, that for each reaction temperature there is a definite maximum hydrogen partial pressure, which must not be exceeded if losses in yield are to be avoided. This is, for example, shown in'the following series of experiments:

Reaction Hydrogen Experiment No. temperapartial g g ture pressure 2 2 4 C. Atm; Percent- 1 420-450 3 l2 2 420-450 2 45 3 420-450 1 70 4 420450 0.5 85

In Germany June 20, 1932 versely the yields with a given hydrogen partialpressure increase with rising temperature, as is, for example, shown in the following series of experiments:

The above results were obtained by employing potassium formate, to which 0.5-1% of caustic alkali had been added. With regard to this second of the above mentioned precautions to be observed according to the invention, it was ascertained that the alkali hydroxide must be present in a greater quantity the lower the temperature selected. A reduction in the speed of reaction caused by the lowering of the temperature can in this manner be simultaneously effectively counteracted by increasing the addition of .caustic alkali. Thus it is, for example, possible by an addition of 5% of free caustic potash to obtain satisfactory results in a sufficiently short time even at temperatures of, for example, 275 0., whereas with an addition of only 1% at the same temperature and the same time of reaction the yields lie below 1%. If, on the other hand, the operation is carried out in the absence of free alkali hydroxide the yields, even under the most favourable temperature and other working conditions, are extraordinarily poor and amount at best to not more than 50% of K2C2O4.

The above mentioned experiments further clearly show that the conversion of potassium formate into potassium oxalate with the liberation of hydrogen is of the nature of a reversible reaction, which is substantially controlled not only by the temperature but also by the hydrogen partial pressure and the simultaneous presence of alkali hydroxide and also the adjustment of these factors. Accordingly the essence and characteristic based on this discovery of the present process for the preparation of potassium oxalate by heating potassium formate, consists therein that, in carrying the process into effect, care is taken to ensure the presence of alkali hydroxide,

in quantities which must be the greater, the lower the temperature selected, and that the hydrogen partial pressure is so adjusted, depending upon the prevailing reaction temperature, that it lies below the equilibrium pressure of the reaction and is constantly maintained below this equilibrium pressure during the reaction.

Working temperatures of about 260-530 C. have been found to be suitable. Above about 530 C. decomposition of the potassium oxalate, increasing with rising temperature, commences with the formation of potash and carbon, while below 260 C. the course of the reaction is too slow and on further lowering the temperature becomes constantly slower. This lowering of the speed of reaction can, it is true, be to a certain extent effectively counteracted in the above mentioned manner, yet it is advisable, as stated above, in general not to use a temperature lower than about 260 C. in order to economize in material.

Within these limits of about 260-530 C. temperatures between about 300-400 C. are the most advantageous, since these temperatures, when observing the above mentioned precautions, not only ensure good yields but also offer the further particular advantage that the potassium oxalate is obtained in a loosely crystalline form which enables it to be easily further worked up.

It follows from the foregoing that the higher the reaction temperature employed the higher can be the hydrogen partial pressure. As shown in the above experiments, however, the hydrogen partial pressure is preferably in general always maintained below 1 atm. This may be effected by carrying out the operation at a. suitably selected reduced pressure. On the other hand the operation may, for example, also be carried into efiect by introducing inert gases and thus, by diluting the hydrogen, adjusting or lowering the partial pressure of the same to the desired value. The latter procedure however only yielded a satisfactory result if the hydrogen formed during the reaction was allowed to escape from the foam-like solid reaction material. This may, for example, be effected by allowing the reaction to take place in a tube, drum or the like apparatus,

in which constant comminution of the foamy mass of reaction material is efiected with the aid of balls, rollers, stirrers or the like, a procedure which is also advantageous in other respects. In this case it is advisable gradually to introduce the potassium formate in the form of a melt into the reaction vessel, which has been previously heated and subjected to reduced pressure, it being in other respects also advantageous gradually to introduce the potassium formate, preferably in a molten condition, into the apparatus.

If hydrogen partial pressures of more than 1 atm. are to be employed, the process can also be carried into effect in such a way that the desired excess pressure is adjusted at the commencement by forcing in hydrogen and then maintaining the pressure at the desired value by allowing corresponding quantities of the hydrogen formed to escape.

In certain cases it is also advantageous to operate in the presence of solid diluents, which are with advantage introduced into the reaction vessel at the commencement of the process. Already formed potassium oxalate or other inert substances may for example be employed for the purpose. These substances also, for example, serve to supplement the aforementioned loosening eiTect of the grinding bodies and stirrers in an advantageous manner.

According to a particular modification of the invention the operation may also be effected by finely atomizing the potassium formate in a molten condition and introducing the fine droplets into an inert gas. Alternatively the potassium formate may be directly atomized into the inert gas, which on being introduced into the apparatus may at the same time serve as a heat carrier. The heated inert gas may also be conversely blown into molten potassium formate and in this way the partial pressure of the hydrogen. be kept low by constantly flushing away the hydrogen formed during the reaction. This action of the inert gas is with advantage promoted by allowing the liberation of the hydrogen to proceed at as low a speed as possible, for example by selecting a sufficiently low reaction temperature.

In a further modification of the process according to this invention it has been ascertained that, when employing inert gases, such as nitrogen, methane, ethylene and the like, which contain oxidizing gaseous compounds, such as oxygen, carbon monoxide, nitrous oxide and the like, i. e. gas constituents, which are in themselves assumed to have no influence on the reaction, very considerably worse results are obtained, than when employing free inert gases.

Thus according to the invention care must be taken to ensure that the inert gases do not contain oxygen and oxygen compounds as impurities or alternatively that the same, if present, are rendered harmless. This can, of course, be effected in a very simple manner by employing at the commencement inert gases free from such admixtures or, if the gases should contain such admixtures, subjecting them before their entry into the reaction vessel to a suitable absorption or other purifying treatment. On the other hand the aforesaid substances can be rendered to a considerable degree harmless by increasing the alkali content of the melt and the reaction temperature to values exceeding those which are in general to'be maintained in the process as hereinbefore mentioned.

The following comparative experiments serve to illustrate the essential feature of this modification of the present invention and the effects obtainable thereby.

1. In a vigorouscurrent of nitrogen, free from oxygen and oxygen compounds as impurities, is passed at 300-350 C. through a melt of potassium formate, containing 3% of free caustic potash, the mass gradually becomes solid with liberation of hydrogen and contains after 1-1 hours about 95% of potassium oxalate.

2. If Experiment 1 is repeated with a nitrogen containing 5% of oxygen as impurity, the other conditions remaining the same, a yield of only 30% of potassium oxalate is obtained. Ingeneral, the higher the oxygen content of the nitro-- gen, the lower is the resulting yield of potassium oxalate.

3. If the nitrogen in Experiment 1 is replaced by carbon monoxide but the other conditions are maintained the same the mass does not become solid at all and contains after 1 hours only about 20% of potassium oxalate, while the residue consists of unchanged potassium formate.

4. If the nitrogen in Experiment 1 is replaced by a nitrogen containing 25% of carbon monoxide as impurity, yields of only about 52% of potassium oxalate are obtained. In this case also the yields in general deteriorate the more carbon monoxide is contained in the nitrogen.

5. It the alkali content of the melt in Experiment 4 is increased to 10% and the temperature at the same time raised to about 400 C. a yield of about 81% of potassium oxalate, i. e. more than double the result of Experiment 4, is obtained in the same time.

The utilization of the discovery underlying the present invention regarding the dependence of the yield on the reaction temperature and hydrogen partial pressure or on the proportion between the hydrogen partial pressure and the equilibrium pressure of the reaction, finally also admits of a further particular modification of the present process, which enables the operation to be carried out also with larger charges, without the reaction becoming too vigorous and consequently rendering the process more or less incapable of being carried out. This particular modification consists in continuously lowering the hydrogen partial pressure within the compass of the aforesaid principles during the course of the reaction by means of the above mentioned procedure. If a potassium formate melt, containing 1% of free caustic potash, is heated at atmospheric pressure, for example to a temperature of 360 C., no or only an extremely slow, conversion and evolution of hydrogen takes place. If, on the other hand, whilst maintaining the temperature, a slowly increasing reduced pressure, which always remains below the equilibrium pressure of the reaction, is produced, 1. e. if the hydrogen partial pressure is continuously decreased during the course of the reaction, the reaction proceeds at the same rate as the reduced pressure rises. It is thus to some extent possible to suck out the hydrogen from the potassium formate and in this way to control the time of the reaction.

What we claim is:

1. A process for the preparation of potassium oxalate by heating potassium formate, characterized in that the process is carried out in the presence of alkali metal hydroxide and that the hydrogen partial pressure is so adjusted in accordance with the prevailing temperature of reaction, that it lies below the equilibrium pressure of the reaction and is constantly maintained below this equilibrium pressure during the reaction.

2. A process according to claim 1, in which the lower the reaction temperature selected between about 260 C. and about 530 C. the greater are the quantities or the alkali hydroxide employed selected between about 0.5% and about 5% based on the potassium formate.

3. A process according to claim 1, in which the hydrogen partial pressure is maintained below one atmosphere.

4. A process according to claim 1, in which the hydrogen partial pressure is continuously lowered during the course of the reaction.

5. A process according to claim 1, in which the operation is effected at temperatures of from 260-530 C.

6. A process according to claim 1, in which the operation is effected at temperatures of from BOO-400 C.

7. A process according to claim 1, in which the potassium formate is gradually introduced into the reaction vessel in a molten condition.

8. A process according to claim 1, in which the potassium formate is gradually introduced into the reaction vessel in a state of fine distribution.

9. A process according to claim 1, in which the operation is efiected in the presence of solid diluents.

10. A process according to claim 1, in which the operation is effected in the presence of ready formed potassium oxalate.

11. A process according to claim 1, in which the reaction material is constantly comminuted during the reaction.

12. A process for the preparation of potassium oxalate by heating potassium formate in the presence of alkali metal hydroxide, in which the hydrogen partial pressure is so adjusted in accordance with the prevailing temperature of reaction,

that it lies during the reaction below the equilibrium pressure of the reaction by introducing inert gases and allowing the hydrogen formed during the reaction to escape from the foam-like solid reaction material.

13. A process according to claim 2, in which care is taken to ensure that the inert gases do not contain oxidizing gaseous impurities selected from the group consisting of oxygen, carbon monoxide and nitrous oxide.

14. A process according to claim 2, in which care is taken to ensure that the inert gases do not contain oxidizing gaseous impurities.

15. A process according to claim 2, in which any oxidizing gaseous impurities present in the inert gases are rendered unharmful.

16. A process according to claim 2, in which the potassium formate is introduced into the process in a state of fine distribution together with a current of inert gases.

17. A process according to claim 2, in which any oxidizing impurities present in the inert gases are rendered harmless by increasing the alkali content of the melt above the normal value between about 0.5% and about 5% based on the alkali formate and by increasing the temperature above the normally applied temperature between about 260 C. .and about 530 C.

MAX ENDERLI. AUGUST SCHRODT. 

